Examples of items for evaluating the safety of batteries such as lithium ion secondary batteries include evaluation of the heat-generating behavior when an internal short circuit has occurred. Examples of tests of such a battery evaluation test include the UL standard for lithium batteries (UL1642), and a guideline from Battery Association of Japan (SBA G1101-1997 Guideline for Safety Evaluation on Secondary Lithium Cells). Such evaluation tests are utilized and the results of the tests are reflected in the development of various batteries. For example, Patent Document 1 discloses that the battery manufacturing process has been improved in order to obtain a high-capacity battery exhibiting excellent high-output characteristics and cycle characteristics and having a high level of safety. In Patent Document 2, it is proposed to set the ratio between the thickness of the heat-resistant porous layer disposed between positive and negative electrodes and the thickness of the side wall of a battery can within a predetermined range, in order to achieve both a high level of safety and high battery characteristics of a lithium secondary battery.
For example, a nail penetration test among the above-described evaluation tests is a test for examining an internal short circuit caused by a nail penetrating through or piercing into a battery from the side face. Due to the nail piercing thereinto, a short circuit portion is generated by the nail between the positive and negative electrodes inside the battery. Consequently, a short circuit current flows through that short circuit portion, thus generating Joule heat. A change in the battery temperature or the battery voltage that is caused by these phenomena is measured in the internal short circuit test.
A crush test is a test for examining an internal short circuit caused by physically deforming a battery using a round bar, a square bar, a flat plate or the like. Specifically, in the crush test, an internal short circuit is caused between the positive and negative electrodes by physical deformation, and a change in the battery temperature or the battery voltage is measured.
Furthermore, other than the nail penetration test and the crush test, a foreign object push-in test in which an electrode group is taken out from a battery, and a foreign object is pushed in from the outside of the electrode group toward the inside is also conducted as the internal short circuit test for a battery.